This is old info from a GM site. Does it still apply?
"When the ECU leaves the factory, it has a certain number attached to it. Every time it is accessed, the number changes. When the ECU is accessed by a dealer, The dealer records the new number before releasing the car and then uploads that new number to GM for its records.
If the car is presented for warranty service on the drivetrain, the dealer first checks the number on the ECU to make sure it matches the number in GM's records. If the numbers do not match, the engine is considered tampered with and no warranty coverage is provided .
The ECU also records time of service and any gaps in service. If it is taken out of service because the new ECU is used, the old ECU will show that gap"
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I don't have a tune but for those who do, it might be worth checking...